In the apparatus disclosed in the above U.S. patent, a first signal is transmitted from the recording unit and is received in the recording transmitter where it is frequency converted to a second signal by a code which is individual to the recording transmitter. Thereafter, the recording transmitter retransmits the coded second signal, which is received by the recording unit, and the information content of the signal is processed and utilized. As disclosed in the above U.S. patent, the aforesaid frequency converter comprises a modulator which modulates the first signal with the code in such a way that a sideband is formed, preferably by starting and stopping the modulator according to a pattern constituting said code.
According to the present invention, the information symbols in the emitted, information-containing signal are such that the lengths of the information symbols are advantageously related to pre-selected frequencies.
The present invention achieves this object in a unique, simple way. The invention also provides an apparatus for extremely reliable transfer of information symbols between the recording transmitter and recording unit.
According to the present invention, the recording transmitter is capable of transmitting a single sideband where the information symbols are related to the frequency of the sideband, where each information symbol is transmitted by means of a single sideband and where the length of each information symbol is detected in the recording unit for evaluating the information symbol. In addition to affording a reliable and accurate transfer of information, these features of the present invention enable the information to be transferred at a faster rate as compared with the aforesaid known apparatus, because a certain information symbol can be followed immediately by a number of the same information symbols, without giving rise to difficulties in evaluating the number of consecutive information symbols of equal value.
In the illustrated embodiment, the invention is applied to a traffic control system. The mobile units in the system are each provided with a recording unit, and the stationary places are each provided with a recording transmitter. The invention, however, can be applied also in reverse order, in which case recording transmitters will be attached to mobile objects and the recording units will be located in certain selected stationary places. As examples of such an embodiment there can be mentioned the recording and sorting of railway cars and load containers, as well as the recording of motor cars.
The invention is described below, with reference to the below-described drawings.